Abstract:
Exotic ants have become invasive in many regions around the world, with variable
ecological impacts. Post-invasion, native ant communities are often found to be depauperate,
though the drivers of minimal coexistence are rarely well known. Myrmica rubra, a Palearctic
Myrmecine ant, is currently expanding its range as an invasive in North America. This
aggressive ant forms dense, patchy local infestations and appears to aggressively displace native
ant fauna. We measured behavioral interactions and rates of recruitment in experimental field
assays pitting native foragers against captive colonies of M. rubra at tuna-jelly or aphid baits in
uninfested areas of Mt. Desert Island, Maine. Behavioral interactions were idiosyncratic with
respect to the native opponent, but M. rubra generally showed significantly higher levels of
recruitment, aggression and displacement of native foragers. Since the absence of a tradeoff
between resource discovery rate and behavioral dominance appears to contribute to invasion
success in other exotic ant species (the dominance-discovery hypothesis), we studied M. rubra
and naturally co-occurring native ants at baits along the invasion front. Myrmica rubra was
consistently faster to discover baits and disproportionately displaced native foragers, confirming
a break in the dominance-discovery tradeoff and providing a plausible proximate mechanism for
native ant exclusion. Finally, we surveyed ant recruitment at baits for 24 hours in August 2004
at four sites with varying M. rubra abundance but found little evidence of temporal niche
partitioning. Taken together, these results indicate competitive superiority by M. rubra with
respect to native ant communities of the northeastern North America and suggest direct
aggression and competitive exclusion at food resources can lead to local native displacement.
